in where clause

The classes Item_func_isnottrue and Item_func_isnotfalse inherited the
implementation of the eval_not_null_tables method from the Item_func
class. As a result the not_null_tables_cache was set incorrectly for
the objects of these classes. It led to improper conversion of outer
joins to inner joins when the where clause of the processed query
contained IS NOT TRUE or IS NOT FALSE predicates. The coverted query
in many cases produced a wrong result set.